The present invention relates to a system for detecting and controlling a knocking generated in an internal combustion engine.
In general, a knocking phenomenon (also called knock) which appears in an internal combustion engine refers to an abnormal combustion due to early ignition of uncombusted air fuel mixture (called end gas) within an engine cylinder and consequently the abnormal combustion appears in a form of resonant vibration in pressure in the engine cylinder (cylinder pressure) having a plurality of inherent frequencies determined by the dimensions of the cylinder (particularly cylinder bore inner diameter) and combusted gas temperatures.
The inherent frequencies fall in a range approximately from 5 to 6 kilohertz or more.
Such pressure vibration propagates in air via a cylinder wall and cylinder block, etc., and is received by a human's hearing organ as an uncomfortable high-frequency sound. This is called a knocking sound.
Such a knocking phenomenon appears remarkably in a case when the engine load is above a constant value and the ignition timing is too advanced at an angle earlier than a normal ignition advance angle value. Such a strong knock causes uncomfortable noise and a strong vibration within the cylinder. Consequently, an abnormal high temperature in an inner part of the cylinder is generated due to a columnar vibration caused by the strong knocking so that the engine suffers damage.
However, a low level of knocking phenomenon itself does not have an adverse effect on the engine body and engine output characteristics. If the ignition timing is advanced and such low level of knocking appears, combustion efficiency of the engine increases so that fuel consumption can be improved. Therefore, it is preferable to allow and leave the engine at an appropriate level of knocking in order to achieve an engine driving condition with an optimum combustion efficiency of the engine in view of improvement of fuel consumption.
To enhance the driving efficiency of the engine and to suppress noise level due to the knock below a predetermined level, it is necessary to control so that the level of knocking matches various engine operating conditions.
Conventional knocking detection and control systems of internal combustion engines are carried out in such a way that knocking generated due to the abnormal combustion of the engine is detected by means of knocking detecting means comprising a pressure sensor and the result of detection, i.e., a level of knocking is compared with a predetermined reference value so that the presence or absence of knocking is determined. In addition, the ignition timing is then adjusted on the basis of the determination result so that the recurrence of knocking is avoided and an appropriate fuel combustion is carried out.
In this case, however, the above-described knocking detecting means not only detects the knocking proper but also detects variations in pressure within each engine cylinder involved in the normal combustion and in pressure involved in acceleration and deceleration of engine. In addition, such background noise caused by variations in pressure is not fixed but fluctuates with time depending on various engine operating conditions. Furthermore, since an output characteristic of the knocking detecting means has deviations, this results in the background noise. The avoidance of recurrence of knocking depends mainly on how accurate the knocking is discriminated from such background noise.
To cope with such a conventional problem as described above, various systems for detecting and controlling the knocking in the engine have been proposed.
Japanese Patent Application Unexamined Open No. Sho 59-39,972 published on Mar. 5, 1984 exemplifies such a system as described above. In the system disclosed in the above-identified document, an average value of the output level of the knocking means for a predetermined interval of a crank angle (for a predetermined angle through which an engine crankshaft has rotated) is used as the background noise level and the background noise level multiplied by k (constant number) is set as a first reference value. The system then compares a maximum value of the output level of the knocking detecting means for the predetermined interval of the crankshaft rotation with the first reference value.
When the maximum value thereof is greater than the first reference value, the system determines the occurrence of knocking and retards the ignition timing by a predetermined retardation angle. In addition, the system compares the maximum value thereof with a predetermined second reference value which is fixed and determines the occurrence of knocking when the maximum value described above is greater than the second reference value, thus adjusting the ignition timing so as to retard it by the retardation angle. In this way, the system controls the ignition timing so as to shift the ignition timing toward retardation with respect to the current timing when the maximum value thereof is greater than the first or second reference value. It should be noted that the mean value of the output level of the knocking detecting means is derived from a value of an output integrated signal of an integration circuit after rectification of an output waveform of the knocking detecting means by means of a rectifier.
However, there is a problem in the system disclosed in the above-identified document. That is to say, since the average value of the output level of the knocking detecting means which is multiplied by the constant number is used as the reference value to be compared, a high level of background noise continues for a long duration even though no knocking occurs in a range of high engine revolution speeds. In addition, since the level of background noise is rapidly changed depending on the engine operating condition, the reference value (first reference value) is set at a high-level value so that it is impossible to discriminate accurately the knocking noise from the background noise. Consequently, accurate avoidance and suppression for recurrence of knocking cannot sufficiently be achieved. Therefore, the maximum output torque of the engine cannot sufficiently be obtained with noisy knock sounds being generated, thus the engine durability is reduced.
On the other hand, Japanese Patent Application Examined Open No. Sho 58-13,749 published on Mar. 15, 1983 exemplifies the other knocking detecting means.
The knocking detecting means disclosed in the above-identified document comprises a knock sensing block including a pressure-responsive sensor installed at a spark plug as a washer thereof, a signal amplifier, a low-pass filter, an average circuit having an envelope processing circuit of knocking signal, a comparator, and an ignition timing control circuit.
In the disclosed system of No. Sho 58-13,749, a signal component having a particular frequency band equal to or more than approximately five or six kilohertz of the detected signal corresponding to the internal cylinder pressure is extracted as a knocking signal and the knocking signal is then processed in a peak value detection circuit to produce an envelope signal. The envelope signal is assumed as a signal corresponding to knocking energy and is directly compared with a predetermined reference value. The knocking level is then controlled to a desired level by advancing or retarding the ignition timing on the basis of the comparison result.
However, there is also a problem in the knocking detecting means disclosed in the above-identified document as will be described in details below.
In general, the above-described particular frequency band component is also included in the output signal of the sensor at the time of no occurrence of knocking, i.e., at a time when fluctuations in combustion pressure do not appear.
FIG. 1 shows an example of power spectra of pressure vibrations within the engine cylinders in the internal combustion engine. It should be noted that although the spectra shown in FIG. 1 are experimental results when the engine having a four cylinder and displacement of 1800 cc is driven with full load and at 4800 rpm, the substantially same result has been observed in the other different engine models by the Applicants' series of experiments.
Hence, in the conventional system disclosed in the latter document, the determination of the knocking level becomes difficult.
Since FIG. 1 shows such knocking as that having a relatively large level, a difference in the power level between those at the time of no occurrence of knocking A and at the time of occurrence of knocking B is equal to or greater than about 10 dB. However, since the difference is substantially two or three decibels in a state where a trace pattern of knocking which provides a boundary between the determinations of presence and absence of knocking, it is extremely difficult to determine an appearance of such pattern of knocking.
In addition, since the output signal level itself of the pressure-responsive sensor is largely changed during no occurrence of knocking due to an initial characteristic and deterioration of aging effect and due to an influence of mechanical vibration system caused by a load on a sensor element surface or fastening torque in a case when the pressure-responsive sensor is attached to the spark plug, such a level change often becomes larger than that of the output signal due to the influence described above during the occurrence of knocking.
Furthermore, according to experiments by the Applicants using several types of engines, the output variations of the pressure-responsive sensor during no occurrence of knocking are three or four times with respect to an engine load with an engine idling or with a wide open throttle valve and are two or three times with respect to the engine speed of 800 rpm or 4800 rpm.
Since in the conventional detecting means disclosed in the latter document, the signal of the pressure-responsive sensor having such a large variation is directly compared with the reference value, an extraneous number of experiments are needed for each pressure-responsive sensor, for each type of engine, and for each engine operating condition in order to achieve a system which can accurately determine the presence or absence of knocking.